1. Field of the Invention
The present invention relates to a projection lens and an optical projector provided with the same for projecting an image formed on a slide, a film or a glass plate or an image displayed by a liquid crystal display in an enlarged image on a screen.
2. Description of the Related Art
A projection lens for a low-cost LCD projector is required to have a small f-number, to be compact and lightweight, to be capable of forming an image having high definition and low distortion, to cause a small chromatic aberration of magnification, to be telecentric, to have a long back focal distance and a short projection distance, and to be inexpensive.
As mentioned in JP2000-137165A, projection optical systems of most optical projectors that project an image formed on a film or a slide, or an image formed by a liquid crystal display in an enlarged view on a screen employ a telecentric zoom lens. However, most zoom lenses are costly.
Requirements for forming high-definition images with low distortion and low chromatic aberration of magnification can be satisfied by a fixed focus projection lens consisting of a plurality of lenses. However, the plurality of lenses increases the internal absorbance of light waves of short wavelengths and hence the transmittance of the projection lens is small. Although a projection lens having an acceptable f-number can be constructed by assembling lenses of lead glass having low internal absorbance with light waves of short wavelengths, such a projection lens, however, is expensive.